JADA Continuing Education
CUTTING EFFICIENCY OF THREE DIAMOND BUR GRIT SIZES
SHARON CRANE SIEGEL, D.D.S., M.S. and
J. ANTHONY VON FRAUNHOFER, M.SC., PH.D.
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ABSTRACT
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Background. Tooth preparation requires safe, efficient and rapid cutting, and diamond burs routinely are used for extracoronal preparation and gross tooth reduction. Coarser-grit diamond burs often are used for gross tooth reduction, with tooth surface finishes being sacrificed for the presumed greater cutting rates, or CRs, of the coarser diamond burs. The authors compared the CRs of medium-, coarse- and super–coarse-grit diamond burs.
Methods. The authors used a self-contained dental treatment system with digitally controlled handpiece speed, torque and water flow rate to cut a machinable glass ceramic cutting substrate with medium-, coarse- and super–coarse-grit diamond burs from the same manufacturer under a load of 147.5 grams (0.9 kilonewton at the bur tip) and a coolant flow rate of 22 milliliters per minute. They made three cuts through 13-millimeter bars of the cutting substrate with six diamond burs of each grit size. They determined CRs as the transection time per millimeter and analyzed CR data by one-way analysis of variance and post hoc Scheffé tests.
Results. The authors found no statistically significant difference in CR (P > .05) between the three diamond bur grit sizes for the first (13 mm) cuts. When they compared the three cuts (39 mm total cut length), they found no difference (P > .05) between CRs for coarse- and super–coarse-grit diamond burs, but they did find that the super–coarse-grit diamond burs cut faster than the medium-grit diamond burs (P < .01).
Conclusion. Differences in CR for the three diamond bur grit sizes are due to the greater decrease in CR for the medium-grit diamond burs (50 percent) compared with the CRs of the coarse- and super–coarse-grit diamond burs (35 percent and 25 percent, respectively) over the total cutting period.
Clinical Implications. Coarser-grit diamond burs may be useful for extensive gross tooth preparations, but dental professionals should be aware of the associated effects of the coarser grit on surface finish, heat generation and enamel damage.
Dentists in the United States favor diamond burs for extracoronal tooth preparation and enameloplasty, while they use carbide burs for intracoronal cutting.1 Dentists in many other countries use dental diamond burs not only for gross tooth reduction but also for a wide variety of intracoronal procedures.2 This difference is accentuated by the availability of a wide range of diamond bur grit sizes; however, there do not appear to be any evidence-based data on the optimal selection of diamond burs for gross tooth reduction. Bur selection and use are complicated by the fact that cutting efficiency, or CE, tends to decrease as burs wear out and as debris accumulates on the bur.3 As a result, dentists may press harder on the tooth to maintain the CE that was possible before the handpiece or bur performance degraded.
A survey we conducted of clinical teaching in U.S. and Canadian dental schools indicated that there is an overall preference in predoctoral dental programs for using medium-grit diamond burs for gross tooth reduction, while coarse-grit diamond bur use predominates in advanced prosthodontic programs.1 The general perception among dentists appears to be that coarser-grit diamond burs cut faster and more efficiently than finer-grit diamond rotary instruments.
We undertook the present study to evaluate the CE of diamond burs of different grit sizes when they are used under controlled handpiece speed, water coolant flow rate and applied load conditions.
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METHODS AND MATERIALS
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We performed the cutting studies using a previously established testing regimen for diamond burs (Figure 1
)4,5 that we modified for a dental treatment system with digitally controlled handpiece speed, torque and water flow rate (KaVo INTRAsept 905, KaVo America Corp.). We used Macor (Corning Inc.), a machinable glass ceramic cutting substrate, because its hardness (Knoop hardness number of 250), elastic modulus (66.9 gigapascals) and thermal properties are comparable with those of dental enamel.4 We performed the cutting studies on 13-millimeter cross-section rectangular bars of Macor at a handpiece loading of 147.5 grams (0.9 kilonewton at the bur tip) and a coolant flow rate of 22 milliliters per minute. Cutting was performed at a bur rotation speed of 160,000 revolutions per minute (2.7 x 103 per second) under a torque of 3 newton centimeters.
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Figure 1. Schematic diagram of cutting test system. Macor is manufactured by Corning Inc. Adapted from Siegel and von Fraunhofer3 with permission of the publisher.
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The cutting instruments we selected were multiuse, conventional medium-grit (856-016), coarse-grit (6856-016) and super–coarse-grit (5856-016) diamond burs from the same manufacturer; we used a 5-mm length of each bur. We used six burs of each type to make three 13-mm cuts through the Macor bars, totaling 39 mm per bur. We determined cutting rates, or CRs, as the time in seconds it took the diamond bur to transect the Macor bar. We analyzed data by one-way analysis of variance with post hoc Scheffé tests at an a priori 
= .05.
All three diamond bur grit sizes showed a decrease in cutting rate with prolonged cutting.
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RESULTS
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The diamond bur CRs (amounts of Macor removed) are summarized in the table and Figures 2 and 3. The table also includes the percentage decrease in CR between Cut 1 and Cut 2 and between Cut 1 and Cut 3. Statistical analysis of the data showed no significant difference (P > .05) between CRs for the three diamond bur grit sizes over Cuts 1 and 2. We did, however, find differences in CRs for the three diamond bur grit sizes over Cut 3 with both the coarse-grit (P < .05) and the super–coarse-grit (P < .01) diamond burs showing greater CRs than the medium-grit diamond burs. We found no difference in CR between the coarse-grit and super–coarse-grit diamond burs (P > .05).
All three diamond bur grit sizes showed a decrease in CR with prolonged cutting. The data in the table
show that compared with Cut 1 there was a decrease in CR of 50.0 percent for the medium-grit diamond burs, 35.3 percent for the coarse-grit diamond burs and 25.0 percent for the super–coarse-grit diamond burs at Cut 3. Statistical analysis of the cutting data showed no difference between the three grit sizes in the CR decrease from Cut 1 to Cut 2 for any of the diamond burs. However, the CR decreased a further 21.4 percent for the medium-grit diamond burs and 14.3 percent for the coarse-grit diamond burs from Cut 2 to Cut 3. In contrast, the super–coarse-grit diamond burs showed no decrease in CR from Cut 2 to Cut 3.
As a consequence of these different decreases in CR with longer cutting times, the super–coarse-grit diamond burs cut faster than the medium-grit diamond burs for Cut 3 (P < .05), while there was no difference in CR between the coarse-grit diamond burs and super–coarse-grit diamond burs (P > .05). When we compared the overall CRs, we found that the super–coarse-grit diamond burs cut faster than the medium-grit diamond burs (P < .01), but there was no difference in CR (P > .05) between coarse-and super–coarse-grit diamond burs over the cutting procedure.
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DISCUSSION
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There has been a resurgence of interest and research activity in dental cutting studies stemming from several developments in contemporary dentistry. One of these is the introduction and commercial exploitation of newer technologies such as laser cutting and air abrasion. While these technologies have undeniable appeal and applicability in many areas of dentistry, the dental hand-piece and bur still dominate in tooth preparation for the general dentist. A second and very important development has been the introduction of different types of burs, disposable burs, a wide variety of finishing instruments and burs designed for specific purposes such as cutting of ceramics, implant abutments and metals, as well as for preclinical teaching. Further, economic pressures require dentists to perform such cutting procedures as caries removal and cavity preparation as rapidly and efficiently as possible without damaging the health of the vital tooth. Finally, the challenges facing the dentist to trim, polish and adjust restorations fabricated from the newer dental biomaterials are increasingly in concert with the advances in dental bio-materials technology.
Until recently, the scientific literature on the CE of dental burs has been sparse and often inconsistent.6 Cutting studies have been limited by numerous technical problems in the past. These include the difficulty of maintaining constant handpiece speed and torque under controlled water spray (cooling) action, as well as the lack of sufficient test materials that mimic dental hard tissues. The continuing research within our laboratory at the University of Maryland Dental School has minimized the inherent variability in cutting studies by adopting a testing regimen that is reproducible and controls most cutting variables.
Sales figures of diamond burs provided by a leading manufacturer (L. Rose, Brasseler USA, personal communication, May 1998) indicate that many dentists believe that coarser-grit diamond burs have a higher CR, as shown by the sales figures to the profession. This manufacturer reported that the relative percentages of sales of coarse-and super–coarse-grit diamond burs to private practitioners (29 percent and 27 percent, respectively) are greater than the sales of medium- and fine-grit diamond burs (24 percent and 20 percent, respectively). In contrast, the relative percentages of medium- and fine-grit diamond burs sold to dental schools (38.4 percent and 34.4 percent, respectively) are greater than the sales of coarse- and super–coarse-grit diamond burs (21.5 percent and 5.7 percent, respectively). Another bur manufacturer (A. Dibiasi, S.S.White Burs Inc., personal communication, February 2000) reported similar preferences for coarser-grit diamond burs among general practitioners. The relative proportions of sales were 73 percent for coarse-grit, 16 percent for medium-grit and 11 percent for fine-grit diamond burs.
The reasons for the differences between burs sold to practitioners and those sold to dental schools may be associated with dental teaching objectives for undergraduate students.
The reasons for the differences between burs sold to practitioners and those sold to dental schools are not documented, but they may be associated with dental teaching objectives for undergraduate students. This conclusion stems from a recent survey of dental school teaching in operative and prosthodontic dentistry, which indicated that medium-grit burs are the favored cutting instrument.1 Teaching objectives encompass such factors as minimizing possible damage to teeth during predoctoral training, obtaining a smoother surface finish and minimizing the clinical procedures necessary to achieve a satisfactory tooth preparation. In support of this philosophy is Ottl and Lauer’s7 observation that greater heating effects are found when cutting with coarser-grit diamond burs. It also has been shown that coarse-grit diamond burs induce 84-micrometer (± 30 µm, standard deviation) cracks in enamel during cutting, and the surface must be finished with fine-grit diamond burs to eliminate these cracks.8 This finding adds support to the dental school teaching recommendations that suggest better finishes and reduced surface damage occur when finer-grit sizes are used.
The present study supports our previous work,4,5 which showed no significant differences in the CRs of coarse- and medium-grit diamond burs from the same manufacturer. The present data show that the CRs of medium-, coarse- and super–coarse-grit diamond burs are comparable over short cutting periods. It is only after prolonged cutting that differences in CR become apparent between the coarse- and the medium-grit diamond burs.
While super–coarse-grit diamond burs have a high CR during longer cutting procedures, the clinician must question whether the increase in CR counterbalances the associated detrimental effects. In particular, the adverse effects on surface finish, heat generation and enamel damage found with coarser-grit diamond burs may not justify the apparent increase in CR.
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CONCLUSIONS
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CE depends on both the diamond bur grit size and the duration of the cutting procedure. Over short cutting periods, medium-, coarse- and super–coarse-grit diamond burs have comparable CRs. With prolonged cutting, however, the CR decreases more slowly with the coarser-grit diamond burs compared with medium-grit diamond burs. We found no difference in CR between coarse- and super–coarse-grit diamond burs.
The clinical implication is that while super–coarse-grit diamond burs promote CE when extensive gross tooth preparation is necessary, it is questionable whether there really is a time savings, given the need to subsequently refine the surface finish and eliminate enamel damage. Thus, coarser-grit diamond burs may be preferred by experienced practitioners during tooth preparations for full-mouth reconstructions or multiunit bridges but not for limited preparative procedures.
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FOOTNOTES
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The authors would like to express their appreciation to Mr. Seth Feldman for his outstanding technical assistance with this study.
Dr. Siegel is an assistant professor, Department of Restorative Dentistry, School of Dentistry, University of Maryland, 666 West Baltimore St., Baltimore, Md. 21201, e-mail "scs001@dental. umaryland.edu". Address reprint requests to Dr. Siegel.
Dr. von Fraunhofer is a professor and the director, Biomaterials Science, Department of Restorative Dentistry, School of Dentistry, University of Maryland, Baltimore.
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REFERENCES
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- Siegel SC, von Fraunhofer JA. Dental burs: what bur for which application? A survey of dental schools. J Prosthodont 1999; 8:258–63.[Medline]
- Kimmel K. Optimal selection and use of rotary instruments for cavity and crown preparations. Dent Echo 1993;63(2):63–9.
- Siegel SC, von Fraunhofer JA. Effect of handpiece load on the cutting efficiency of dental burs. Machining Sci Technol J 1997;1:1–13.
- Siegel SC, von Fraunhofer JA. Assessing the cutting efficiency of dental diamond burs. JADA 1996;127:763–72.[Abstract/Free Full Text]
- Siegel SC, von Fraunhofer JA. Dental cutting with diamond burs: heavy-handed or light touch? J Prosthodont 1999;8:3–9.[Medline]
- Bleiholder R, Rosenstiel S, Gegauff A, Martello J, McCafferty R. A laboratory performance test for dental rotary instrument. J Dent Res 1987;66:1746.
- Ottl P, Lauer HC. Temperature response in the pulpal chamber during ultrahigh-speed tooth preparation with diamond burs of different grit. J Prosthet Dent 1998;80:12–9.[Medline]
- Xu HH, Kelly JR, Jahanmir S, Thompson VP, Rekow ED. Enamel subsurface damage due to tooth preparation with diamonds. J Dent Res 1997;76:1698–706.[Abstract/Free Full Text]
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